maven编译报错 -source 1.5 中不支持 lambda 表达式

在用maven编译项目是由于项目中用了jdk 1.8, 编译是报错  -source 1.5 中不支持 lambda 表达式，Google找到这篇解决方案，记录一下：

编译时报如下错误：

[ERROR] COMPILATION ERROR : 
[INFO] -------------------------------------------------------------
[ERROR] AAA\.jenkins\workspace\BBB\CCC.java:[73,46] 错误: -source 1.5 中不支持 diamond 运算符
[ERROR]   (请使用 -source 7 或更高版本以启用 diamond 运算符)
[ERROR] AAA\.jenkins\workspace\BBB\DDD.java:[38,33] 错误: -source 1.5 中不支持 lambda 表达式
[ERROR]   (请使用 -source 8 或更高版本以启用 lambda 表达式)

奇怪的是我的 Jenkins 构建机器上只安装了 JDK 8，为什么还会说不支持 diamond 和 lambda 呢？在 Google 大神的指引下，在 ​​Maven Compiler 插件介绍​​​ 里面找到了答案：​​Alsodefaultis1.5anddefaultis1.5,Mavenwith.​​

原来 Maven Compiler 插件默认会加 -source 1.5 及 -target 1.5 参数来编译（估计是为了兼容一些比较老的 ​​Linux​​ 服务器​​操作系统​​，它们通常只有 JDK 5），而我们的代码里使用了 JDK 7/8 的语法。解决办法在​​这里​​：

<project>
  [...]
  <build>
    [...]
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.2</version>
        <configuration>
          <source>1.8</source>
          <target>1.8</target>
        </configuration>
      </plugin>
    </plugins>
    [...]
  </build>
  [...]
</project>

Setting the -source and -target

Sometimes when you may need to compile a certain project to a different version than what you are currently using. The javac can accept such command using -source and -target. The Compiler Plugin can also be configured to provide these options during compilation.

For example, if you want to use the Java 8 language features (-source 1.8) and also want the compiled classes to be compatible with JVM 1.8 (-target 1.8), you can either add the two following properties, which are the default property names for the plugin parameters:

<project>
  [...]
  <properties>
    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>
  </properties>
  [...]
</project>

or configure the plugin directly:

<project>
  [...]
  <build>
    [...]
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.6.0</version>
        <configuration>
          <source>1.8</source>
          <target>1.8</target>
        </configuration>
      </plugin>
    </plugins>
    [...]
  </build>
  [...]
</project>

Note: Merely setting the target option does not guarantee that your code actually runs on a JRE with the specified version. The pitfall is unintended usage of APIs that only exist in later JREs which would make your code fail at runtime with a linkage error. To avoid this issue, you can either configure the compiler's boot classpath to match the target JRE or use the ​​Animal Sniffer Maven Plugin​​​ to verify your code doesn't use unintended APIs. In the same way, setting the sourceoption does not guarantee that your code actually compiles on a JDK with the specified version. To compile your code with a specific JDK version, different than the one used to launch Maven, refer to the ​​Compile Using A Different JDK​​ example.

​​http://maven.apache.org/components/plugins/maven-compiler-plugin/examples/set-compiler-source-and-target.html​​